Foreign affairs have been traditionally seen as an exclusive competence of central governments. However, over the last 30 years, European paradiplomacy has been progressively developing not least because of the institutional opportunities that the Union composite constitutional order provides for the participation of the regional tier in its decision-making processes. The present paper examines how the European multilevel systems have allowed for the creation of such âsub-national constitutional spaceâ enabling their constituent units to be active in the international arena. It does so by examining the treaty-making powers of the sub-state entities, the mechanisms that allow their participation in the foreign policy making of the central government and the implementation of the international treaties. Finally, it focuses on their autonomous external representation at the EU level. It argues that, despite conventional wisdom, States do not enjoy a monopoly of competences in the area of foreign affairs